Driving circuit to avoid reverse current for soft switching DC motor

ABSTRACT

A driving circuit includes a power supply, an input capacitor, a Hall sensor, a first amplifier, a second amplifier, a full-bridge driver circuit, and a first operational amplifier. The input capacitor is coupled to the power supply. The input end of the first amplifier and the second amplifier is coupled to the output end of the Hall sensor. The control end of the full-bridge driver circuit is coupled to the output end of the first amplifier and the output end of the second amplifier. The first operational amplifier includes a first input end for receiving a first reference voltage and a second input end coupled to the first output end of the full-bridge driver circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a soft switching DC motor driver and arelated driving circuit for avoiding reverse current, and moreparticularly, to a soft switching DC motor driver and a related drivingcircuit for avoiding reverse current by using operational amplifiers forfixing output voltages.

2. Description of the Prior Art

A DC motor driver is a necessary power transformation device in modernindustries and information society. The DC motor is capable oftransforming electricity into kinetic energy required for drivingdevices. Conventional motors include DC motors, AC motors, and steppingmotors. DC motors and AC motors are often applied in products notrequiring delicate manipulations. For example, blades of an electric fanare rotated with a DC motor or an AC motor. As the technology of digitalproducts grows, a rotation rate of a DC motor or an AC motor is requiredto be faster and faster. However, with a high rotation rate of a motor,the current of the motor cannot be consumed completely. The unconsumedand therefore remaining currents reversely flow to a corresponding powersupply. This scenario leads to damages of controllers and drivers of themotor. Therefore, the damages caused by reverse current under a highrotation rate of the motor have to be avoided.

Please refer to FIG. 1, which is a diagram of a prior art soft switchingDC motor driver 10. The soft switching DC motor driver 10 comprises apower supply 12, an input capacitor C1, a hall sensor 16, a firstamplifier AMP1, a second amplifier AMP2, and a full-bridge drivercircuit 14. The power supply 12 is utilized for generating an inputvoltage Vin. The input capacitor C1 is coupled to the power supply 12. Avoltage difference between both terminals of the input capacitor C1 is asupply voltage VDD. The Hall sensor 16 has a first output end 162 forgenerating a first timing control signal H+, and a second output end 164for generating a second timing control signal H−. The first amplifierAMP1 has a first input end 102 coupled to the first output end 162 ofthe Hall sensor 16, a second input end 104 coupled to the second outputend 164 of the Hall sensor 16, a first output end 106, and a secondoutput end 108. The first amplifier AMP1 is utilized for amplifyingsignals inputted from the first input end 102 and the second input end104. The second amplifier AMP2 has a first input end 112 coupled to thesecond output end 164 of the Hall sensor 16, a second input end 114coupled to the first output end 162 of the Hall sensor 16, a firstoutput end 116, and a second output end 118. The second amplifier AMP2is utilized for amplifying signals inputted from the first input end 112and the second input end 114. The full-bridge driver circuit 14 has aninput end 142 coupled to the power supply 12 and the input capacitor C1,and the voltage at the input end 142 is the supply voltage VDD. Thefull-bridge driver circuit 14 comprises a first switch SW1, a secondswitch SW2, a third switch SW3, a fourth switch SW4. The inductor L ismodeled as a motor. The first switch SW1 has a control terminal 132coupled to the first output end 106 of the first amplifier AMP1, aninput end 134 coupled to the power supply 12 and the input capacitor C1,and an output end 136 for generating a first output voltage Vout1. Thesecond switch SW2 has a control terminal 152 coupled to the secondoutput end 108 of the first amplifier AMP1, an input end 154 coupled toground, and an output end 156 coupled to the output end 136 of the firstswitch SW1. The third switch SW3 has a control terminal 172 coupled tothe first output end 116 of the second amplifier AMP2, an output end 174coupled to the power supply 12 and the input capacitor C1, and an outputend 176 for generating a second output voltage Vout2. The fourth switchSW4 has a control terminal 192 coupled to the second output end 118 ofthe second amplifier AMP2, an input end 194 coupled to ground, and anoutput end 196 coupled to the output end 176 of the third switch SW3.The inductor L has a first terminal 182 coupled to the first switch SW1and the second switch SW2, and a second terminal 184 coupled to thethird switch SW3 and the fourth switch SW4. The soft switching DC motordriver 10 further comprises a protecting diode D1 coupled to the powersupply 12 and the input capacitor C1 for protecting the power supply 12and for preventing reverse current, which may burn down the entireintegrated circuit. The first switch SW1, the second switch SW2, thethird switch SW3, and the fourth switch SW4 may be metal-oxidesemiconductor transistors, the first switch SW1 and the third switch SW3are P-type metal-oxide semiconductor transistors, and the second switchSW2 and the fourth switch SW4 are N-type metal-oxide semiconductortransistors. The first switch SW1, the second switch SW2, the thirdswitch SW3, and the fourth switch SW4 may also be bipolar-junctiontransistors, the first switch SW1 and the third switch SW3 are npnbipolar-junction transistors, and the second switch SW2 and the fourthswitch SW4 are pnp bipolar-junction transistors.

Please refer to FIG. 1 and FIG. 2. FIG. 2 is a waveform diagram of thesignals shown in FIG. 1. The soft switching DC motor driver 10 controlsthe switches of the full-bridge driver circuit 14 by soft switchingdriving techniques. Therefore, waveforms of the first output voltageVout1 and the second output voltage Vout2 are trapezoidal waves formitigating high-frequency voltage pulse of the soft switching DC motordriver 10 during transitions and a voltage impulse caused by reversecurrent. Therefore, low noises of the soft switching DC motor driver 10is achieved and the reliability of the soft switching DC motor driver 10is also enhanced.

Please refer to FIG. 2 again. The first timing control signal H+ and thesecond timing control signal H−, which are outputted by the Hall sensor16, are utilized for controlling the first switch SW1, the second switchSW2, the third switch SW3, and the fourth switch SW4. When the firsttiming control signal H+ is low and the second timing control signal H−is high, the first switch SW1 and the fourth switch SW4 are turned on,and the second switch SW2 and the third switch SW3 are turned off. Aninductance current flows I_(L) from the first output voltage Vout1 tothe second output voltage Vout2, at this time, the first output voltageVout1 is high, and the second output voltage Vout2 is low. During thetransition of the first timing control signal H+ and the second timingcontrol signal H−, the second switch SW2 and the fourth switch SW4 areturned on, and the first switch SW1 and the third switch SW3 are turnedoff. The inductance current I_(L) weakens gradually through the secondswitch SW2 and the fourth switch SW4. At this time, the waveform of thefirst output voltage Vout1 descends linearly during a transient whereasthe waveform of the second output voltage Vout2 ascends linearly duringthe transient. When the first timing control signal H+ is high and thesecond timing control signal H− is low, the second switch SW2 and thethird switch SW3 are turned on, and the first switch SW1 and the fourthswitch SW4 are turned off. The inductance current I_(L) flows from thesecond output voltage Vout2 to the first output voltage Vout1, at thistime, the first output voltage Vout1 is low, and the second outputvoltage Vout2 is high.

Please refer to FIG. 3, which is a waveform diagram of the signals ofFIG. 1 when a high rotation rate of the soft switching DC motor resultsin a voltage impulse. During the first stage, the first timing controlsignal H+ is low, and the second timing control signal H− is high. Theinductance current I_(L) flows from the first output voltage Vout1 tothe second output voltage Vout2, at this time, the first output voltageVout1 is high, and the second output voltage Vout2 is low. During thesecond stage and the transition of the first timing control signal H+and the second timing control signal H−, the inductance current I_(L)weakens gradually through the second switch SW2 and the fourth switchSW4. However, since the rotation rate of the soft switching DC motor ishigh, the inductance current I_(L) cannot be weakened to be zero afterthe transition of the switches. Therefore, during the third stage, theinductance current I_(L) flows reversely to the supply voltage VDDthrough the second switch SW2 and the fourth switch SW4, and charges theinput capacitor to result in a voltage impulse. As shown in FIG. 3, themagnitude of the voltage impulse depends on the magnitude of the reversecurrent flowing into the input capacitor C1 and the capacitance of theinput capacitor C1. During the fourth stage when the first timingcontrol signal H+ turns to high and the second timing control signal H−turns to low, the inductance current I_(L) flows from the second outputvoltage Vout2 to the fist output voltage Vout1, at this time, the firstoutput voltage Vout1 is low, and the second output voltage Vout2 ishigh.

Please refer to FIG. 4, which is a diagram illustrating the flow of theinductance current I_(L) during the first stage shown in FIG. 3. Duringthe first stage, the first amplifier AMP1 and the second amplifier AMP2are saturated, and the first switch SW1 and the fourth switch SW4 arefully on. The inductance current I_(L) flows from the first outputvoltage Vout1 to the second output voltage Vout2.

Please refer to FIG. 5, which is a diagram illustrating the flow of theinductance current I_(L) during the second stage shown in FIG. 3. Duringthe second stage, a feedback loop of both the first amplifier AMP1 andthe second amplifier AMP2 begins working, then the first output voltageVout1 descends linearly whereas the second output voltage Vout2 ascendslinearly. At this time, the inductance current I_(L) weakens gradually,and the first switch SW1 and the fourth switch SW4 are turned on. Theinductance current I_(L) continues to flow from the first output voltageVout1 to the second output voltage Vout2.

Please refer to FIG. 6, which is a diagram illustrating the flow of theinductance current I_(L) during the third stage shown in FIG. 3. Duringthe third stage and when the first output voltage Vout1 falls below −0.7volts, since diode of the second switch SW2 is turned on, the feed backloop due to the first amplifier AMP1 is broken, and the switch SW1 nolonger provides current to the soft switching DC motor driver 10,therefore, the first switch SW1 is turned off whereas the second switchSW2 is fully on. Similarly, when the second output voltage Vout2increases over (VDD+0.7) volts, the fourth switch SW4 is turned offwhereas the second switch SW2 is turned on. At this time, the inductancecurrent I_(L) charges the input capacitor C1 to increase the supplyvoltage VDD and to result in a voltage impulse.

Please refer to FIG. 7, which is a diagram illustrating the flow of theinductance current I_(L) during the fourth stage shown in FIG. 3. Duringthe fourth stage, since the inductance current I_(L) has weakened to bezero, the second switch SW2 and the third switch SW3 are turned onwhereas the first switch SW1 and the fourth switch SW4 are turned off.The inductance current I_(L) flows from the second output voltage Vout2to the first output voltage Vout1.

For a DC motor having a low rotation rate, controlling the switches ofthe full-bridge driver circuit 14 with the soft switching drivingtechniques may mitigate the high frequency voltage pulse of the softswitching DC motor driver 10 during the transition and the voltageimpulse caused by the reverse current. However, in modern applications,the rotation rate of a modern motor is ever increasing. When therotation rate of the motor exceeds a limit, the inductance current I_(L)has not weakened to be zero after the transition of the switches. Atthis time, the inductance current I_(L) flows reversely to the supplyvoltage VDD through the second switch SW2 and the fourth switch SW4, andcharges the input capacitor C1 to result in the voltage impulse.Therefore, the controller and the driver of the soft switching DC motordriver 10 would be damaged or burned down, the power supply 12 may beburned down also, and the reliability and the effective operationalrange of the system of the soft switching DC motor driver 10 would bedegraded.

SUMMARY OF THE INVENTION

The claimed invention provides a soft switching DC motor driver to avoidreverse current. The soft switching DC motor driver comprises a powersupply for generating an input voltage, an input capacitor coupled tothe power supply, a Hall sensor having a first output end for generatinga first timing control signal, and a second output end for generating asecond timing control signal, a first amplifier having a first input endcoupled to a first output end of the Hall sensor, a second input endcoupled to a second output end of the Hall sensor, a first output end,and a second output end, the first amplifier being utilized foramplifying the first input signal and the second input signal of thefirst amplifier, a second amplifier having a first input end coupled tothe second output end of the Hall sensor, a second input end coupled tothe first output end of the hall sensor, a first output end, and asecond output end, the second amplifier being utilized for amplifyingthe first input signal and the second input signal of the secondamplifier, a full-bridge driver circuit having an input end coupled tothe power supply and the input capacitor, a first control terminalcoupled to the first output end of the first amplifier, a second controlterminal coupled to the second output end of the first amplifier, athird control terminal coupled to the first output end of the secondamplifier, a fourth control terminal coupled to the second output end ofthe second amplifier, a first output end for generating a first outputvoltage, and a second output end for generating a second output voltage,and a first operational amplifier having a first input end for receivinga first reference voltage, a second input end coupled to the firstoutput end of the full-bridge driver circuit, and an output end coupledto a control terminal of the first amplifier, the first operationalamplifier for controlling output voltages of the first amplifier.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a prior art soft switching DC motor driver.

FIG. 2 is a waveform diagram of the signals shown in FIG. 1.

FIG. 3 is a waveform diagram of the signals of FIG. 1 when a highrotation rate of the prior art soft switching DC motor results in avoltage impulse.

FIG. 4 is a diagram illustrating the flow of an inductance currentduring the first stage shown in FIG. 3.

FIG. 5 is a diagram illustrating the flow of the inductance currentduring the second stage shown in FIG. 3.

FIG. 6 is a diagram illustrating the flow of the inductance currentduring the third stage shown in FIG. 3.

FIG. 7 is a diagram illustrating the flow of the inductance currentduring the fourth stage shown in FIG. 3.

FIG. 8 is a diagram of a soft switching DC motor driver of the presentinvention.

FIG. 9 is a waveform diagram of the signals shown in FIG. 8 under a highrotation rate of the soft switching DC motor of the present invention.

FIG. 10 is a diagram illustrating the flow of an inductance currentduring the third stage shown in FIG. 9.

FIG. 11 is a diagram illustrating the flow of the inductance currentduring the fifth stage shown in FIG. 9.

DETAILED DESCRIPTION

Please refer to FIG. 8, which is a diagram of a soft switching DC motordriver 80 of the present invention. The soft switching DC motor driver80 comprises a power supply 12, an input capacitor C1, a Hall sensor 16,a first amplifier AMP1, a second amplifier AMP2, a full-bridge drivercircuit 14, a first operational amplifier OP1, a second operationalamplifier OP2, a first reference voltage generator 84, and a secondreference voltage generator 86. The power supply 12 is for generating aninput voltage Vin. The input capacitor C1 is coupled to the power supply12. A voltage difference between both terminals of the input capacitorC1 is a supply voltage VDD. The Hall sensor 16 has a first output end162 for generating a first timing control signal H+, and a second outputend 164 for generating a second timing control signal H−. The firstamplifier AMP1 has a first input end 102 coupled to the first output end162 of the Hall sensor 16, a second input end 104 coupled to the secondoutput end 164 of the Hall sensor 16, a first output end 106, and asecond output end 108. The first amplifier AMP1 is for amplifyingsignals inputted at the first input end 102 and the second input end104. The second amplifier AMP2 has a first input end 112 coupled to thesecond output end 164 of the Hall sensor 16, a second input end 114coupled to the first output end 162 of the Hall sensor 16, a firstoutput end 116, and a second output end 118. The second amplifier AMP2is for amplifying signals inputted at the first terminal 112 and thesecond terminal 114. The full-bridge driver circuit 14 has an input end142 coupled to the power supply 12 and the input capacitor C1, and thevoltage at the input end 142 is the supply voltage VDD. The full-bridgedriver circuit 14 comprises a first switch SW1, a second switch SW2, athird switch SW3, a fourth switch SW4, and an inductor L. The firstswitch SW1 has a control terminal 132 coupled to the first output end106 of the first amplifier AMP1, an input end 134 coupled to the powersupply 12 and the input capacitor C1, and an output end 136 forgenerating a first output voltage Vout1. The second switch SW2 has acontrol terminal 152 coupled to the second output end 108 of the firstamplifier AMP1, an input end 154 coupled to ground, and an output end156 coupled to the output end 136 of the first switch SW1. The thirdswitch SW3 has a control terminal 172 coupled to the first output end116 of the second switch SW2, an input end 174 coupled to the powersupply 12 and the input capacitor C1, and an output end 176 forgenerating a second output voltage Vout2. The fourth switch SW4 has acontrol terminal 192 coupled to the second output end 118 of the secondamplifier AMP2, an input end 194 coupled to ground, and an output end196 coupled to the output end 176 of the third switch SW3. The inductorL has a first input end 182 coupled to the first switch SW1 and thesecond switch SW2, and a second input end 184 coupled to the thirdswitch SW3 and the fourth switch SW4.

Please refer to FIG. 8 again. A first input end 802 of the firstoperational amplifier OP1 is for receiving a first reference voltageVref1. The magnitude of the first reference voltage Vref1 is (VDD+VS),and VS is a tiny positive voltage. A second input end 804 of the firstoperational amplifier OP1 is for receiving the first output voltageVout1. An output end 806 of the first operational amplifier OP1 is forgenerating a voltage V1 to control the output stage of the firstamplifier AMP1. Under normal operations, the potential of the firstoutput voltage Vout1 is lower than the potential of the first referencevoltage Vref1, therefore, the first operational amplifier OP1 is turnedoff so that it does not affect the control of the first amplifier AMP1over the full-bridge driver circuit 14. When the potential of the firstoutput voltage Vout1 is higher than the first reference voltage Vref1, afeedback circuit of the first operational amplifier OP1 begins working,transmits current signals for controlling the output stage of the firstamplifier AMP1, and fixes the potential of the first output voltageVout1 at (VDD+VS) with a negative feedback mechanism. Similarly, a firstinput end 812 of the second operational amplifier OP2 is for receivingthe second reference voltage Vref2, whose potential is (VDD+VS). Asecond input end 814 of the second operational amplifier OP2 is forreceiving the second output voltage Vout2. An output end 816 of thesecond operational amplifier OP2 is for generating a voltage V2 tocontrol the output stage of the second amplifier AMP2. Under normaloperations, the potential of the second output voltage Vout2 is lowerthan the voltage of the second reference voltage Vref2, therefore, thesecond operational amplifier OP2 is turned off so that it does notaffect the control of the second amplifier AMP2 over the full-bridgedriver circuit 14. When the potential of the second output voltage Vout2is higher than the potential of the second reference voltage Vref2, afeedback circuit of the second operational amplifier OP2 begins working,transmits current signals for controlling the output stage of the secondamplifier AMP2, and fixes the potential of the second output voltageVout2 at (VDD+VS) with the negative feedback mechanism.

Please refer to FIG. 9, which is a waveform diagram of the signals shownin FIG. 8 under a high rotation rate of the soft switching DC motordriver 80 of the present invention. During the first stage, and when thefirst timing control signal H+ is high whereas the second timing controlsignal H− is low, the inductance current I_(L) flows from the firstoutput voltage Vout1 to the second output voltage Vout2. At this time,the first output voltage Vout1 is high, and the second output voltageVout2 is low. During the second stage and the transition of the firsttiming control signal H+ and the second timing control signal H−, theinductance current I_(L) releases energy through the second switch SW2and the fourth switch SW4. During the third stage and when the potentialof the second output voltage Vout2 is raised to (VDD+VS), the secondoperational amplifier OP2 begins working after detecting the raisedsecond output voltage Vout2, controls the output stage of the secondamplifier AMP2, and further controls the control terminals of the thirdswitch SW3 and the fourth switch SW4. At last, a negative feedbacksystem is generated by a loop gain of the second operational amplifierOP2, and the potential of the second output voltage Vout2 is fixed at(VDD+VS). During the fourth stage, the feedback loop of the secondoperational amplifier OP2 is off, and the second amplifier AMP2 controlsthe third switch SW3 and the fourth switch SW4 instead.

Please refer to FIG. 10, which is a diagram illustrating the flow of theinductance current I_(L) during the third stage shown in FIG. 9. Duringthe third stage, and when the potential of the second output voltageVout2 is raised to (VDD+VS), the second operational amplifier OP2 beginsworking after detecting the raised second output voltage Vout2, controlsthe output stage of the second amplifier AMP2 through the voltage V2,and further controls the third switch SW3 and the fourth switch SW4.Since a negative feedback loop is generated by the loop gain of thesecond operational amplifier OP2, the connection between the first inputend 812 and the second input end 814 of the second operational amplifierOP2 is virtual shorted so that the potential of the second outputvoltage Vout2 is fixed at (VDD+VS). At this time, the inductance currentI_(L) releases energy through the second switch SW2 and the fourthswitch SW4 until the inductance current is completely dissipated. Thenegative feedback circuit turns off the second operational amplifier OP2while the potential of the second output voltage Vout2 is lower than(VDD+VS), then the operations of the system of the soft switching DCmotor driver 80 return to normal.

Please refer to FIG. 11, which is a diagram illustrating the flow of theinductance current I_(L) during the fifth stage shown in FIG. 9. Duringthe fifth stage, and when the first output voltage Vout1 is raised to(VDD+VS), the first operational amplifier OP1 begins working afterdetecting the raised first output voltage Vout1, controls the outputstage of the first amplifier AMP1 through the voltage V1, and furthercontrols the first switch SW1 and the second switch SW2. Since anegative feedback loop is generated by a loop gain of the firstoperational amplifier OP1, the connection between the first input end802 and the second input end 804 of the first operational amplifier OP1is virtual shorted so that the potential of the first output voltageVout1 is fixed at (VDD+VS). At this time, the inductance current I_(L)releases energy through the second switch SW2 and the fourth switch SW4until the inductance current I_(L) is dissipated. The negative feedbackcircuit turns off the first operational amplifier OP1 while thepotential of the first output voltage Vout1 is lower than (VDD+VS), andthe operations of the system of the soft switching DC motor driver 80return to normal.

The abovementioned embodiments are presented merely for describing thepresent invention, and in no way should be considered to be limitationsof the scope of the present invention. The potentials of the firstreference voltage Vref1 and the second reference voltage Vref2 are both(VDD+VS) in the abovementioned embodiments, though, are not limited to(VDD+VS) in the present invention. However, the potentials of the firstreference voltage Vref1 and the second reference voltage Vref2 aredefinitely higher than the supply voltage VDD for not resulting inerrors of the soft switching DC motor driver of the present invention.Moreover, the first switch SW1, the second switch SW2, the third switchSW3, and the fourth switch SW4 are not limited to be metal-oxidesemiconductor transistors or bipolar-junction transistors, and otherelements may also be utilized for implementing the abovementionedswitches.

From the above descriptions, the present invention provides a softswitching DC motor driver 80 for avoiding reverse current. By virtualshorting the connection between the first input end 802 and the secondinput end 804 of the first operational amplifier OP1, the potential ofthe first output voltage Vout1 is fixed at (VDD+VS) until the inductancecurrent I_(L) is dissipated for preventing the inductance current I_(L)from reversely flowing to the supply voltage VDD thereby causing damageto the controllers and the drivers of the soft switching DC motor driver80. The present invention may also be applied in DC motors having a lowrotation rate or a high rotation rate for effectively preventing avoltage impulse and enhancing the reliability and the effectiveoperational range of the system of the soft switching DC motor driver.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A soft switching DC motor driver to avoid reverse current comprising: a power supply for generating an input voltage; an input capacitor coupled to the power supply; a Hall sensor having a first output end for generating a first timing control signal, and a second output end for generating a second timing control signal; a first amplifier having a first input end coupled to the first output end of the Hall sensor, a second input end coupled to the second output end of the Hall sensor, a first output end, and a second output end, the first amplifier being utilized for amplifying the first input signal and the second input signal of the first amplifier; a second amplifier having a first input end coupled to the second output end of the Hall sensor, a second input end coupled to the first output end of the hall sensor, a first output end, and a second output end, the second amplifier being utilized for amplifying the first input signal and the second input signal of the second amplifier; a full-bridge driver circuit having an input end coupled to the power supply and the input capacitor, a first control terminal coupled to the first output end of the first amplifier, a second control terminal coupled to the second output end of the first amplifier, a third control terminal coupled to the first output end of the second amplifier, a fourth control terminal coupled to the second output end of the second amplifier, a first output end for generating a first output voltage, and a second output end for generating a second output voltage; and a first operational amplifier having a first input end for receiving a first reference voltage, a second input end coupled to the first output end of the full-bridge driver circuit, and an output end coupled to a control terminal of the first amplifier, the first operational amplifier for controlling output voltages of the first amplifier.
 2. The soft switching DC motor driver of claim 1, further comprising: a second operational amplifier having a first input end for receiving a second reference voltage, a second input end coupled to the second output end of the full-bridge driver circuit, and an output end coupled to a control terminal of the second amplifier, the second operational amplifier for controlling output voltages of the second amplifier.
 3. The soft switching DC motor driver of claim 1, further comprising: a protector coupled to the power supply and the input capacitor for protecting the power supply.
 4. The soft switching DC motor driver of claim 3 wherein the protector is a diode.
 5. The soft switching DC motor driver of claim 1, further comprising: a first reference voltage generator coupled to the first input end of the first operational amplifier for generating the first reference voltage.
 6. The soft switching DC motor driver of claim 2, further comprising: a second reference voltage generator coupled to the first input end of the second operational amplifier for generating the second reference voltage.
 7. The soft switching DC motor driver of claim 1 wherein the full-bridge driver circuit comprises: a first switch having a control terminal coupled to the first output end of the first amplifier, an input end coupled to the power supply and the input capacitor, and an output end for generating the first output voltage; a second switch having a control terminal coupled to the second output end of the first amplifier, an input end coupled to ground, and an output end coupled to the output end of the first switch; a third switch having a control terminal coupled to the first output end of the second amplifier, an input end coupled to the power supply and the input capacitor, and an output end for generating the second output voltage; a fourth switch having a control terminal coupled to the second output end of the second amplifier, an input end coupled to ground, and an output end coupled to the output end of the third switch; and an inductor having a first input end coupled to the first switch and the second switch, and a second input end coupled to the third switch and the fourth switch.
 8. The soft switching DC motor driver of claim 7 wherein the first switch, the second switch, the third switch, and the fourth switch are metal-oxide semiconductor (MOS) transistors.
 9. The soft switching DC motor driver of claim 7 wherein the first switch and the third switch are P-type metal-oxide semiconductor transistors, and the second switch and the fourth switch are N-type metal-oxide semiconductor transistors.
 10. The soft switching DC motor driver of claim 7 wherein the first switch, the second switch, the third switch, and the fourth switch are bipolar junction transistors (BJT).
 11. The soft switching DC motor driver of claim 7 wherein the first switch and the third switch are npn bipolar junction transistors, and the second switch and the fourth switch are pnp bipolar junction transistors. 